A fiber-reinforced thermoplastic resin-made stampable sheet (hereinafter referred to as "FRTP stampable sheet") that is one of fiber-reinforced-resin composite materials capable of being press-molded, is increasing its demand especially in structural components for a vehicle, as an alternative material (substituent) for a metal material, in view of its superior product strength, processability, mass productivity, light-weightness, corrosion resistance, elastic recoverability, energy absorbability and the like.
The FRTP stampable sheet can broadly be divided into a unidirectional reinforced sheet, a bidirectional reinforced sheet, a non-oriented reinforced sheet, and a composite sheet (whose structure is a combination of the aforesaid sheets), depending upon a difference in a manner of orientation of glass long-fibers as a reinforcement. Among these, the composite sheet having a combined structure of the non-oriented reinforced sheet and the unidirectional reinforced sheet has become to be used, in substitution for the conventional metal material, as a component which is required to have mechanical strength in a specific direction (for example, in a longitudinal direction in the case of a bumper beam for a vehicle, or the like), and which is also required to have light-weightness and degree of-freedom of configuration.
Such FRTP stampable sheet, which has particularly superior mechanical strength in the specific direction, is manufactured as follows. That is, glass long-fiber bundles are shaken down spirally, whereby a laminated produce of the glass long-fiber bundles (hereinafter referred to as "non-oriented fiber layer"), which are so arranged as to have no directional property, is prepared. Further, a plurality of glass long-fiber bundles are arranged in parallel relation to each other, whereby an arranged product of glass long-fiber bundles (hereinafter referred to as "unidirectional fiber layer"), which are so arranged as to have a directional property in a specific direction, is prepared. Then, a glass fiber mat is prepared from said laminated product (non-oriented fiber layer) and said arranged product (unidirectional fiber layer) by mechanically intertwining them with each other by a needle punch or the like. Finally, the glass fiber mat as a reinforcement is impregnated with thermoplastic resin to obtain the FRTP stampable sheet.
Here, in forming the unidirectional fiber layer, in order to increase tensile strength of a product in a specific direction, that is, tensile strength of a product in the orientation direction of the glass long-fiber bundles forming the unidirectional fiber layer (hereinafter referred to as "reinforcing direction"), a relatively high tension is applied to each of the glass long-fiber bundles, such that they are arranged in parallel relation to each other and in a linear manner as far as possible.
However, the conventional FRTP stampable sheet has the following problems, because molding flowability in the reinforcing direction at stamping molding (press molding) is insufficient.
1 For example, in the case of being molded into a complicated configuration having recesses for mounting stays at both ends of a bumper beam, a size of blank (cut article of the FRTP stampable sheet for molding) and a charge pattern (how to place the heated blank within a mold, at press molding) are complicated, making it difficult to obtain high productivity.
2 In the case that an elongated article is molded, underfill is liable to occur at longitudinal ends.
3 At molding, abnormal orientation (weld) is liable to occur in the glass long-fibers forming the unidirectional fiber layer, making it difficult to produce a molded article which is small in variation of breaking load and which is superior in reliability.
Furthermore, the conventional FRTP stampable sheet has insufficient impregnation ability of the thermoplastic resin Into the glass long-fiber bundles forming the unidirectional fiber layer. Accordingly, the molded article, which utilizes the conventional FRTP stampable sheet, has the following problems.
1 A superior tensile strength can be achieved in the reinforcing direction. In the case, however, where the FRTP stampable sheet is applied to an element or component which requires superior tensile strength and superior compressive strength, such as a bumper beam, compressive strength in the reinforcing direction referred to above Is still insufficient in view of safety.
2 Since the glass fibers are liable to be exposed to a surface of a molded article, the article is inferior in fine view and is deteriorated in feel or touch. And, there may be a case where a stab is suffered by the exposed glass fiber at handling.
Accordingly, an object of the invention is to provide an FRTP stampable sheet and its molded article which have solved the above-discussed problems.